1. Field of the Invention
This invention relates generally to the field of projection display systems. More particularly, this invention relates to a projection display system where a modulated laser beam is scanned across a surface by rotating mirrors and where areas of the surface illuminated by the laser form pixels of a video image.
2. Description of Related Art
The present invention is an improvement to a projection display device disclosed in a co-pending application, 7229/53235, U.S. patent application Ser. No. 08/854,872. In that device, a modulated laser is scanned across a viewing surface by vertical and horizontal scanning rotating mirrors. A video signal is used to modulate the intensity of the laser. The laser beam is first reflected from the vertical scanning mirror to position the beam for scanning along a particular horizontal scan line. The beam is then reflected from the horizontal scanning mirror to scan the beam across the viewing surface.
Such a system is an improvement over known projection displays because the resolution and aspect ratio of the scanned image can be dynamically adjusted. Also, by using intense laser light sources, images can be projected on large viewing surfaces to create, for example, outdoor displays.
That system suffers from the problem that the rotation rate for the horizontal scanning mirror must be very high in order to properly display a video signal. For example, to display an NTSC video signal with 330 horizontal scan lines per field at a rate of 30 fields per second, a sixteen-sided rotating mirror must rotate over 600 times per second.
High rotation rates make this system susceptible to wear and vibration. Also, the radial dimension of the horizontal mirror must be small, so that the centripetal forces on the mirror do not distort its reflective surfaces. Use of small mirrors requires precise alignment of the optical components. Construction and maintenance of this precise alignment increases the cost and reduces the reliability of the system.
Rotation rates can be reduced by using additional laser sources and scanning them in parallel, as was suggested in the above mentioned application. Additional lasers, however, add cost to the system and increase the complexity of the controlling electronics. This is a particular problem where high-intensity lasers must be used since these lasers are costly and require complicated high-energy power supplies.